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Sunday, June 20, 2010

Venus Tessera Lander Concept

Note: A previous version of this entry had the lander crashing onto the surface at 32 km/second -- that should have been 32 km/hour (or 9 m/second * 3600 seconds in an hour / 1000 m in a kilometer).

The Decadal Survey is exploring 25 mission options (at last report) for possible inclusion in the next decade of planetary missions. (See the complete list of concepts in this blog entry.) Each mission is initially scoped out by a team at one of NASA's centers or John Hopkin's Applied Physics Laboratory to understand the technical requirements. An independent firm then assesses the likely costs of the mission. The results of two concept studies have recently been published as part of the proceedings of the 7th International Planetary Probe Workshop. In this entry, I'll describe the Venus Intrepid Tessera lander (VITAL) concept, and in the next entry I'll describe the Venus mobile explorer concept.

In many ways, the proposed tessera lander sounds much like the Venus SAGE lander currently in competition for the next New Frontiers mission slot. While details probably differ between the two proposed missions, what we learn from the from the VITAL concept probably also applies to the SAGE proposal

Broadly speaking, the Venusian surface is covered by extensive plains (which make up the largest portion), large shield volcanoes, the tessera. The latter are continent-sized regions of highly deformed, folded terrain. The origin of the tessera is unknown, although there is speculation that they may represent the oldest terrain on Venus. Measurement of the composition of the tessera is a high scientific priority. (By contrast, the SAGE mission would go to recently identified terrain on the flanks of a volcano that may be among the youngest terrain on Venus.)

In the past, it's been assumed that the tessera were off limits to landers because the steep terrain would prevent a safe landing. For this concept study, it was assumed that the slope might be as high as 30 degrees and that the lander might partially sit on a a large rock, creating further tilt. To deal with these conditions, the probe's pressure vessle is mounted above a heavy outer ring that lowers the center of gravity to provide stability in case of extreme tilt of up to 72.7 degrees. The lander uses drag plates to slow its descent through the dense lower atmosphere and hits the surface with a speed of 32 km/hour. The thermal system is designed to allow the probe to function during the one hour descent and for two hours on the surface.

Example sampling area for the Raman/LIBS instruments and context imaging.

A decade ago, it was assumed that high quality compositional measurements of the Venus surface would require a complicated sampling mechanism that would deliver samples to instruments inside the probe through an airlock. Today's mission concepts instead rely on lasers to illuminate or melt the surface materials with the results analyzed via spectrometry. (The Mars Science Laboratory's ChemCam will use this technique at Mars.) Using lasers and spectrometers greatly simplifies the probe design since only a window is needed to access the surface. The laser will operate in two modes. In a low power mode, it will illuminate the surface for analysis by a Raman spectrometer. At higher power, the surface material is vaporized for laser induced breakdown spectrometry (LIBS) and the resulting plasma is spectrally analyzed. Analysis is carried out across a 0.86 m row of spots each 0.3 mm approximately two meters from the lander. A camera will take high resolution context images of the sampling area.

Another key goal of the mission is analysis of the atmospheric composition during the descent and on the surface. A neutral mass spectrometer and a tunable laser spectrometer will perform these tasks. Another set of instruments will measure the physical charateristics of the atmosphere such as temperature and pressure during the descent.

Two camera systems will provide context images of the landing site. The descent camera will take a series of nested panchromatic images as the probe nears the surface. The panoramic imager will take color and near infrared images of the surface in four directions to image a total of 240 degrees of the horizon.

Comparison to the SAGE proposal: Only limited information on the SAGE mission has been publicly released (see this blog entry for a summary). At the summary level, the two missions seem very similar. They would carry nearly identical instrument sets, although the SAGE mission apparently would have an arm that could dig a trench to allow the laser to shoot targets 3-10 cm below the surface. The SAGE lander would be designed to survive for three hours on the surface.

The two missions are so similar that if the SAGE mission is selected for the next New Frontiers mission, many aspects of its design likely could be reused for a tessera mission. No cost estimate is given for the VITAL proposal, but it would seem that if the SAGE mission can be flown within the cost cap of the New Frontiers mission (~$650M for the spacecraft and another ~$550M for other costs), then the VITAL mission could in a similar ballpark, and perhaps less if it can reuse substantial portions of the SAGE design.

The lander does NOT hit the surface at 32 km/second. In fact, the lander touches down at less than 9 m/s, very similar to the Soviet Venera landers. The complete report, as well as all the mission concept study reports are available upon request through the National Research Council. If you are interested in finding out more, please read the complete studies.

About Me

You can contact me at futureplanets1@gmail.com with any questions or comments.
I have followed planetary exploration since I opened my newspaper in 1976 and saw the first photo from the surface of Mars. The challenges of conceiving and designing planetary missions has always fascinated me. I don't have any formal tie to NASA or planetary exploration (although I use data from NASA's Earth science missions in my professional work as an ecologist).
Corrections and additions always welcome.